The alchemical triads: sulphur, mercury, salt. Each triad is headed by a principle (composed colour) that is the synthesis of two non-composed, elemental colours: sulphur (red-orange-yellow); mercury (yellow-green-blue); and salt (blue-violet-indigo). The first two triads are in a sense 'perfect': the principial colour is in fact the proper mean term between the elemental colours: red + yellow = orange; yellow + blue = green. But the third triad presents an anomaly: it 'should' be blue + red = violet. The red that 'should' be there—the impulse that is equivalent to the culminating note of the octave, is not there. Indigo is. Diagram by A. Cheak, 2014.